1. Field of the Invention
The present invention relates to a method of modifying an electrostatic chuck and a plasma processing apparatus.
2. Description of the Related Art
When a wafer etching process is performed using plasma, a reaction by-product is deposited on the inner wall surface of a processing chamber. When the thickness of the deposited reaction by-product reaches a predetermined thickness or greater, a portion of the deposited reaction by-product may peel off and turn into particles that may have adverse effects on the wafer etching process. Accordingly, a cleaning process for removing the deposited reaction by-product from the chamber and a process for adjusting the atmosphere within the chamber are periodically performed.
Recently, in efforts to reduce costs, the so-called waferless dry cleaning (also referred to as “WLDC” hereinafter) technique for cleaning a chamber without the use of a cleaning wafer is becoming widespread (see e.g., Patent Document 1). According to this technique, a waferless cleaning process is performed, and as a result, an electrostatic chuck is exposed to plasma during the cleaning process.
Particularly, when an etching process is performed on a silicon-based film, WLDC may be performed using a gas mixture of SF6 gas and O2 gas or a gas mixture of CF4 gas and O2 gas to remove a silicon-based reaction by-product deposited on the inner wall surface of the chamber. When the electrostatic chuck is exposed to plasma generated from such fluorine gases, the surface of the electrostatic chuck may be gradually fluorinated. For example, in a case where the surface of the electrostatic chuck is formed by a yttrium oxide (Y2O3) spray coating, the electrostatic chuck surface may be fluorinated and modified to yttrium fluorine (YF). YF has a lower volume resistivity compared to Y2O3. Accordingly, when the electrostatic chuck surface is modified to YF, a current flows more easily on the electrostatic chuck surface. Thus, when the electrostatic chuck surface is Y2O3, the electrostatic chuck generates a Coulomb force to attract a wafer, whereas when the electrostatic chuck surface is modified to YF, the electrostatic chuck also generates a Johnsen-Rahbek force to attract the wafer so that the electrostatic attraction force of the electrostatic chuck for attracting the wafer is increased. As a result, a torque exerted on a support pin upon separating the wafer from the electrostatic chuck (referred to as “pin torque” hereinafter) gradually increases and separation of the wafer from the electrostatic chuck may become more difficult. In turn, the wafer may be prone to bombardment-related damage or cracking upon separation from the electrostatic chuck.
In this respect, a technique is known for preventing such bombardment-related damage and cracking of the wafer by adjusting discharge conditions for separating the wafer according to an increase in the electrostatic attraction force (see e.g., PCT Japanese Translation Patent Publication No. 2008-519431).
However, when discharge conditions for separating the wafer are adjusted according to an increase in the electrostatic attraction force, a discharge time upon wafer separation may be increased and adjustment operations may have to be repeatedly performed. As a result, throughput may decrease and productivity may be degraded.
With respect to the above-mentioned problems, if the electrostatic chuck surface can be prevented from being fluorinated, the electrostatic attraction force may be prevented from increasing, the discharge time for separating the wafer may not have to be increased, adjustment operations as described above may not have to be performed, and productivity may be improved as a result.
Accordingly, there is a demand for a technique for preventing the surface of the electrostatic chuck from being fluorinated.